I know how to put pictures on here from the internet with the [/img] thing but how can i put the pictures on my computer on the site?
You can't really do that directly from your computer. For that you have to go to photobucket.com or other image/video hosting websites and host your pictures there, then you can link them to here.
You will need to "host" them somewhere on the web. I use "Photobucket.com" to host my pics. Just go there, and set up a free account, then upload your pictures there. PM if you have any trouble.